We studied the effects of two insulin mixtures on glucose levels and plasma free insulin in 12 type I diabetic patients previously rendered normoglycaemic using an artificial pancreas (Biostator). Mixture Iwas made with 24.5 +/- 1.7 I.U. of isophane protamine insulin (Insulatard Nordisk), mixed with 9.8 +/- 0.78 I.U. of short-acting soluble insulin (velosulin Nordisk). Mixture II consisted of identical doses of an insulin zinc suspension (Monotard Novo) with soluble insulin (Actrapid Novo). 30 minutes after injection, all patients had a standard breakfast and blood samples for glycaemia and insulinemia were taken for the next 6 hours. Plasma glucose levels were significantly lower after the injection of mixture I at 30 and 180 minutes (p 0.01) and at 45, 60, 75, 90, 105, 150, 210 and 240 minutes (p 0.05). Free insulin levels were higher after mixture I, although no statistically significant difference was found at any point. We conclude that mixing insulin zinc suspensions in the same syringe with soluble insulin results in an impairment of the quick action of the soluble component.